Hi,
I'll try to answer some of the considerations you'll need to watch out for:
Expansion will be driven by growth / pressure in each of the following areas:
Compute - This will be an expensive area to upgrade as it will probably require an additional server. You'll need to consider enabling EVC mode on your cluster if the model you end up buying contains a CPU from a later family than you currently use. It may have more cores etc unbalancing your cluster layout. You could pool the older servers in one Datacenter while keeping the newest servers together in another to keep similar servers within each cluster but it creates a disparity between the datacenters. You may well end up retiring the older servers to a test/dev capacity and replacing them altogether in the 3-5 year timeframe. I don't find that CPU is the driver for expansion in my experience.
Memory - The server should cope with memory upgrades very readily. This is the cheapest (unless you've maxed out your memory slots with lower capacity memory cards!) and easiest upgrade option. You need to monitor your memory usage with ESXTOP and see how much active memory is in use and avoid oversizing your VMs accordingly. The same can be said for any of the elements here in terms of monitoring.
Network - You've gone with 1Gb network and lots of them. Unless you've a 10 Gb core this is the most cost effective option. In time 10Gb switches will come down in price and it may be a worthwhile option to consider this but only if the VMs are heavily utilizing the 1Gb links and you invest in a 10Gb core along the way for other reasons. Distributed vSwitches make best use of 10Gb with NetIOC but you need to be on a suitable license level of vSphere.
Storage - Another expensive to upgrade option. You are usually looking at a second disk shelf, disks and sometimes vendor licensing to address / replicate the extra capacity. You don't state if your allocation at the storage level or VM level is thin or thick so the 2.5 TB free space could be fairly static or easily grown into. Snapshots also use this space. Make sure you've set alarms for datastore snapshot growth and monitor growth carefully to plan ahead and get sufficient storage in place before it is needed. Beside's growth you may also find you need additional performance, with extra spindle power above and beyond what is needed for capacity.
With all the recent 5.5 announcements, by the time you are looking to upgrade the area above you will have lots of options from Dell and VMware to give you cost effective choices, especially in the area of networking and storage, time you're next upgrade carefully to get the best return on your investment as technology always gets more powerful and therefore cheaper the longer you wait.
Hope this helps!
Mike